United States Patent Application Publication No. 2011/0021298, published Aug. 17, 2010, and U.S. Pat. No. 7,775,921, granted Jan. 27, 2011 describe ring-type tensioners comprising a housing, a plunger-accommodating hole in the housing, a plunger movable in, and protruding from, the plunger-accommodating hole for applying tension to a chain, and a radially expansible elastic ring surrounding the plunger and cooperating with teeth on the outer peripheral surface of the plunger to restrict retracting movement of the plunger.
The rings of the prior art have circular cross-sections. That is, the cross-section of the ring material taken on a plane parallel to the axis of the ring is circular. Because of its circular cross-section, the ring comes into line contact with the wall of the groove in which it is situated. Line contact causes high contact pressure, resulting in excessive wear.
Wear of the elastic ring, in turn, can result in instability in the control of chain tension, and in a condition in which the ring is caught between the plunger and the housing, a phenomenon known as “bit-locking.”
If the plunger protrudes excessively, due to thermal contraction of the engine or to thermal expansion of the chain, excessive tension can be generated in the chain. The condition of excessive tension in the chain can continue because the engagement of the teeth of the plunger with the ring prevents the plunger from being set back.
The plunger is biased in the advancing direction both by a plunger-biasing spring and by the pressure of oil supplied to an oil chamber formed by the housing and the plunger.
If air enters the oil chamber of the tensioner while the engine is stopped, the hydraulic pressure in the oil chamber can become incapable of preventing retraction of the plunger. Therefore, when the engine is started, excessive setback movement of the plunger in reaction to the force applied to the plunger by the chain can result in the generation of a flapping noise in by the chain.
In the prior art ring-type tensioners, when the plunger is installed in, or removed from, the tensioner housing, the elastic ring is expanded manually. Manual expansion is difficult to carry out smoothly because, it is necessary for an individual to use fingers to hold both ends of the elastic ring.
It is also desirable to prevent the plunger from jumping out of the plunger-accommodating hole of the tensioner housing during assembly and transportation of the tensioner.
Because of the problems described above, there is a need for a ring-type tensioner that is less subject to wear at the locations at which the ring contacts the plunger and the housing, and to prevent bit-locking caused by wear. There is also a need for a ring-type tensioner that can avoid excessive chain tension, eliminate the generation of flapping noises by the chain, and securely prevent the plunger of the tensioner from jumping out of the tensioner housing, and for a ring tensioner that can be more easily assembled and maintained.